Heavy objects, such as platform decks, platform jackets and bridge elements are transported by sea on ships or barges. The object is transferred from and to the ship or the barge by hoisting cranes, the object being pushed or pulled, or by means of rolling transport where wheels or rollers are employed.
Another known method is to transfer objects by ballasting the ship or the barge and moving it into a passage under the object, whereupon the ship or the barge is deballasted, thus causing it to be raised and leaving the object lying on the ship or the barge. The object may be unloaded in a similar manner by deballasting the ship or the barge, thus causing it to be lowered, with the object supported by lower-lying supports on the side of the ship or the barge.
These known methods of transporting heavy objects at sea work well as long as the object does not require to be turned, and as long as the level differences which have to be obtained by the ballasting/deballasting are relatively small. When using a ship or a barge, however, it is not possible to turn an object, such as a mast or a platform jacket, which is so large that it has to be transported horizontally, but which has to be turned into a vertical position before being installed in position.
U.S. Pat. No. 5,111,764 describes a method for recovering the substructure of an offshore platform. A floating barge is used comprising a ballastable hull having a deck suitable for receiving the substructure in a substantially horizontal position and including ballastable floats disposed above the deck to right and to left of the intended location for the substructure. The hull and the floats are ballasted so that although it does not sink, the barge moves down into the water and tilts into a position where its deck can be moved against one of the faces of the substructure. The barge is fixed to the substructure, the substructure is disconnected from the sea bed, ballast is removed from the barge to return it to a position where it is floating and carrying the substructure, and the barge is moved to the desired location for unloading the substructure. The invention is applicable to recovering substructures of large dimensions. With the method and barge according to U.S. Pat. No. 5,111,764 it is thereby possible to a certain extent to transport heavy objects at sea, and to turn the object in the water. The degree of attachment and rotational movement which are achieved, however, are limited to the attachment and rotation.